Vacuum insulated current transformer



Oct. 18, 1960 J. E. JENNINGS m-AL 2,957,150

VACUUM INSULATED CURRENT TRANSFORMER Filed Feb. 11, 1957 l9; 8 a 3 Z 23 /NVENTOR8 JO E MME TT JENNINGS HUGH C. ROSS MJW their AT TORNE Y United States Patent VACUUM INSULATED CURRENT TRANSFORMER Jo Emmett Jennings and Hugh 'C. Ross, San Jose, Calif.,

assignors to Jennings Radio Manufacturing Corporation, San Jose, Calif a corporation of California Filed Feb. 11, 1957, Ser. No. 639,388

3 Claims. (Cl. 336-174) Our invention relates to electrical transformers, and particularly to a vacuum insulated current transformer.

One of the objects of our invention is the provision of a vacuum insulated transformer characterized by close spacing of the primary inductor and secondary element.

Another object of the invention is the provision of a high potential vacuum insulated transformer of small size and light weight.

Still another object is the provision of a substantially fireproof vacuum insulated transformer.

A still further object is the provision of a vacuum insulated transformer in which the secondary element may be easily replaced by another possessing a different current rating.

Another object is the provision of a vacuum insulated transformer capable of measuring with equal ease either open or shielded type circuits.

A further object is the provision of a vacuum insulated transformer usable over a wide range of voltages without increasing the size of the primary inductor or secondary element.

A still further object is the provision of a vacuum insulated transformer capable of handling test voltages approximately five times as high as the rated working voltage of the primary inductor.

Another object is the provision of a vacuum insulated transformer which may quickly and easily be dismantled into three separate component parts, and just as easily reassembled into a cooperative combination.

Another object is the provision in a current transformer of the character described of a hermetically sealed and evacuated all glass envelope having electrostatic shield means enclosed therein.

A still further object is the provision of a vacuum insulated transformer in Which heat is efliciently dissipated.

The invention possesses other objects some of which with the foregoing will be brought out in the following description of the invention. We do not limit ourselves to the showing made by the said description and the drawings, since We may adopt variant forms of the invention within the scope of the appended claims.

Referring to the drawings:

The figure is a vertical half-sectional View partly in elevation and showing our Vacuum insulated transformer approximately one-half actual size.

In the measurement of large, low potential alternating currents it is common practice to employ a device known as a Current Transformer. The primary conductor of this device is inserted in series with a circuit conductor, and by measuring the proportional voltage and current induced in the secondary of the transformer, it is possible to :indicate the current flowing through the circuit conductor.

Measurement of currents in high potential conductors however, presents difiiculties because of the inherent limitations in conventional transformers, which require wide spacing between the primary and secondary conductors, and a large amount of insulation therebetween to render them safe under high potential operation. As

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electrical potentials are increased, the spacing and amount of insulation must be correspondingly increased. As the spacing is increased, however, the ability of the transformer to perform its intended function is reduced, thus destroying the efiiciency of the conventional current transformer and rendering it unsuitable for the measurement of high potential currents.

To eliminate the disadvantages inherent in conventional current transformers, we have provided a current transformer characterized by close spacing of the primary inductor and secondary element to provide high efliciency; and effective vacuum insulation interposed between the primary and secondary, rendering our transformer safe and useful for both low and high potential current measurement.

Broadly considered, the vacuum insulated current transformer of our invention comprises an evacuated dielectric annular envelope in which the centrally disposed passageway therethrough receives the primary inductor of the transformer. The primary inductor is preferably removably mounted on the envelope within the passage, and provides means for serially connecting the transformer into an alternating circuit.

Inductively related to the primary inductor is an annular coil constituting the secondary element of the transformer. The secondary element or coil exteriorly encircles and is conveniently supported on the envelope. The evacuated envelope thus interposes vacuum insulation between the primary inductor and secondary element to withstand high potentials and eliminate corona loss. Alternating current caused to flow through the primary inductor induces a proportional alternating voltage and current in the secondary element. Connecting the secondary element with an appropriately calibrated voltmeter or ammeter thus gives a direct reading of the potential or current flow in the circuit being metered. For test purposes, an electrostatic shield may conveniently be hermetically enclosed within the envelope between the primary inductor and the outer envelope Wall.

More specifically, the vacuum insulated transformer of our invention comprises an elongated dielectric envelope having an annular cross-section taken at right angles to its long axis. The envelope is formed by inner and outer concentric cylindrical glass tubes 2 and 3 respectively, integrally joined hermetically at corresponding ends to provide an axially disposed central passageway 4 through the envelope. Evacuation of the hermetically sealed all-glass envelope is effected through the tubulation 5.

Encircling the enevelope intermediate its ends and integrally formed in the outer wall 3 thereof is a glass bead 6. The bead is preferably disposed closer to one end than to the other, and provides a stop for the midway positioning of the secondary element of the transformer. This comprises an annular coil 7 wound on a flanged cylindrical hub 8 forming a snug slip fit with the outer wall 3 of the envelope. When positioned and supported by the envelope bead 6, as shown, the coil preferably lies midway between the ends of the envelope. Suitably calibrated galvanometer means 9, connected electrically to the coil by appropriate leads 11, indicates the potential or current fiow in the energized coil.

The coil encircles and is inductively related to a circuit conductor or primary inductor 12, passing through the central passageway 4 of the envelope, and conveniently comprises a cylindrical copper rod removably supported on the envelope within the passageway by dielectric bushings 13 adjacent opposite ends thereof. The bushings are formed from heat-resistant synthetic resinous material, and are provided with a multiplicity of grooves 14 in the inner and outer peripheral surfaces. The grooves permit the circulation of cooling air through the passage- 7 potential and current flow in the primary inductor.

way 4 to carry away heat generated by the transformer. In this regard, it will be noted that the large surface area' of the enevolpe is particularly suited to the dissipation of heat into the surrounding atmosphere.

Connection. of. the primary inductor into an alternatiing circuit results in the secondary element or coil being inductively energized. The induced potential and current fiow in the secondary element is proportional to the The specific operating ratios may be varied as desired merely by using variously rated coils. A properly calibrated .galvanometer means, operatively connected to the secondary element, may thus be utilized to indicate the potential or. current flow in the primary inductor.

It will sometimes become necessary, due to installation requirements, to substitute an established circuit conductor for the primary inductor shown. In that event, the bushings 13 and primary inductor are removed, and the circuit conductor passed through the passage. An insulating sheath on the circuit conductor will have no appreciable effect upon the operation of the transformer.

Manufacture of our vacuum insulated transformer to meet the specifications of the National Electrical Code requires that each transformer be tested at a voltage approximately five times the rated working voltage of the primary inductor. For example, our vacuum insulated transformer designed to operate in a 15 kv. circuit, must be tested at a voltage of approximately 75 kv. rms. Translated into peak values, this would mean a peak voltage of about 106,000 volts.

Because of the limited dielectric strength of the glass, it becomes necessary to protect the envelope against destructive electrostatically imposed strain at these high peak test voltages. This is efiectively accomplished by a pair of electrostatic shields 16 and 17 hermetically sealed within the envelope. Each of the sheilds may be a true Faraday shield having a plurality of slits in its periphery, or it may be a tubular metallic coating bonded to the inner surface of the glass envelope.

This coating may be achieved as follows; in the embodiment illustrated, a mid-portion of the outer peripheral surface of the inner glass tube 2, and the inner surface of the tube 3 on each side of center are roughened, as by an abrasive or chemical spray. Powdered metallic particles in paste form are spread over the roughened area of each glass tube and heated to coalesce the particles into a unitary layer 18 fused to the roughened glass. Silver, platinum and gold are some of the metals which may conveniently be fused to the glass tubes. For greater stability at the higher test potentials, the fused metallic layer 13 on each glass tube may be electroplated with a relatively more stable metallic layer 19. In the drawing, the thickness of each layer is exaggerated for clarity.

The shields are coaxial and are proportioned in length to extend on both sides of the coil. Both shields thus lie wholly within the magnetic field projected from the centrally disposed primary inductor. Since the field density is constantly varying, an intermittent electrostatic charge is induced on the shields.

The inner shield 17 surrounding glass tube 2 is electrically connected by a lead 21 with the inner end of terminal post 22, hermetically sealed in and extending through the envelope wall. The outer end of the post is electrically connected by lead 23 with the primary inductor. The shield 17 is thus raised to the same electrical potential as the primary inductor when the device is interposed in an alternating circuit, and the formation of an arc between the shield 17 and the primary inductor is prevented.

The outer shield 16 is provided with a terminal post 24, hermetically sealed in and extending through the glass wall 3. The inner end of the terminal post 24 is preferably integrally united to the shield within the enevelope, while the outer end of the post is connected to ground by lead 26.

During operation, the electrostatic shield 16 is charged intermittently by the varying magnetic field, and the charge is channeled to ground by lead 26. T he electro static strainwhich could otherwise rupture the envelope is thus prevented from developing.

I claim:

1. A vacuum insulated current transformer comprising an evacuated dielectric envelope having an inner cylin drical wall and an outer concentric cylindrical wall spaced outward from the inner wall and end wall means at the ends of said walls hermetically connecting said walls whereby an annular evacuated space is formed between said walls and an elongated axial passageway is formed within said inner cylindrical wall and extending through said envelope, a cylindrical conductor constituting the primary inductor of the transformer removably received within and extending through said elongated axial passage through said envelope and in spaced relation within said inner cylindrical wall, dielectric bushing means received over said primary inductor in concentric relation therewith, a coil slideably received and adjustably positionable over the outer cylindrical wall of the envelope in concentric relation therewith and in inductive relation to said primary inductor and separated therefrom by said evacuated dielectric envelope and constituting the secondary element of the transformer, and a first electrostatic shield means encircling and abutting said inner cylindrical wall of the envelope and having means electrically connecting said shield and said primary inductor whereby the primary inductor and said shield will carry an equal potential and a second electrostatic shield means abutting and within the outer cylindrical wall of the envelope and having ground means connected thereto and extending throughand externally of said outer cylindrical wall, said first electrostatic shield means being coaxially disposed about said primary inductor and said second electrostatic shielding means being coaxially disposed about said first shield means and in spaced relation thereto.

2. A vacuum insulated current transformer according to claim 1 in which said first and second electrostatic shield means comprises a conductive metallic coating bonded to the surface of the respectively associated envelope walls.

3. A vacuum insulated current transformer according to claim 1 wherein said outer cylindrical dielectric wall of the evacuated envelope has formed therein an outwardly protruding portion intermediate the ends at a position to form a stop means for positioning said coil forming the secondary at a middle position along said elongated envelope.

References Cited in the file of this patent UNITED STATES PATENTS Germany Sept. 10, 1935 

